AK+LibC+LibCore: Have fewer implementations of days_in_month

Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/a7a18b478e9
Pull-request: https://github.com/SerenityOS/serenity/pull/3307

AK/Time.cpp

@@ -42,4 +42,13 @@ int day_of_year(int year, unsigned month, int day)
     return day_of_year;
 }
 
+int days_in_month(int year, unsigned month)
+{
+    ASSERT(month >= 1 && month <= 12);
+    if (month == 2)
+        return is_leap_year(year) ? 29 : 28;
+
+    bool is_long_month = (month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12);
+    return is_long_month ? 31 : 30;
+}
 }

AK/Time.h

@@ -35,6 +35,9 @@ namespace AK {
 // can be negative.
 int day_of_year(int year, unsigned month, int day);
 
+// Month starts at 1. Month must be >= 1 and <= 12.
+int days_in_month(int year, unsigned month);
+
 inline bool is_leap_year(int year)
 {
     return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
@@ -169,6 +172,7 @@ inline bool operator!=(const TimespecType& a, const TimespecType& b)
 }
 
 using AK::day_of_year;
+using AK::days_in_month;
 using AK::is_leap_year;
 using AK::timespec_add;
 using AK::timespec_add_timeval;

Libraries/LibC/time.cpp

@@ -60,7 +60,6 @@ char* ctime(const time_t* t)
     return asctime(localtime(t));
 }
 
-static const int __days_per_month[] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
 static const int __seconds_per_day = 60 * 60 * 24;
 
 static void time_to_tm(struct tm* tm, time_t t)
@@ -85,16 +84,13 @@ static void time_to_tm(struct tm* tm, time_t t)
     tm->tm_hour = remaining / 60;
     tm->tm_year = year - 1900;
     tm->tm_yday = days;
-    tm->tm_mday = 1;
-    if (is_leap_year(year) && days == 59)
-        ++tm->tm_mday;
-    if (is_leap_year(year) && days >= 59)
-        --days;
+
     int month;
-    for (month = 0; month < 11 && days >= __days_per_month[month]; ++month)
-        days -= __days_per_month[month];
-    tm->tm_mon = month;
-    tm->tm_mday += days;
+    for (month = 1; month < 12 && days >= days_in_month(year, month); ++month)
+        days -= days_in_month(year, month);
+
+    tm->tm_mon = month - 1;
+    tm->tm_mday = days + 1;
 }
 
 static time_t tm_to_time(struct tm* tm, long timezone_adjust_seconds)

Libraries/LibCore/DateTime.cpp

@@ -71,12 +71,7 @@ unsigned DateTime::weekday() const
 
 unsigned DateTime::days_in_month() const
 {
-    bool is_long_month = (m_month == 1 || m_month == 3 || m_month == 5 || m_month == 7 || m_month == 8 || m_month == 10 || m_month == 12);
-
-    if (m_month == 2)
-        return is_leap_year() ? 29 : 28;
-
-    return is_long_month ? 31 : 30;
+    return ::days_in_month(m_year, m_month);
 }
 
 unsigned DateTime::day_of_year() const